A material sheet comprising a consolidated stack of monolayers of a unidirectionally drawn polymer is known from EP 1627719 A1. This publication discloses a multilayered material sheet comprising a plurality of unidirectional monolayers consisting of ultra high molecular weight polyethylene and essentially devoid of bonding matrices, whereby the draw direction of two subsequent monolayers in the stack differs. The monolayers of the multilayered material disclosed in EP 1627719 A1 are produced by positioning a plurality of tapes of ultra high molecular weight polyethylene adjacent to each other whereby adjacently positioned tapes overlap at least partly along their side edges. Without the overlap the known multilayered material cannot be produced.
Although the multilayered material sheet according to EP 1627719 A1 shows a satisfactory ballistic performance, this performance can be improved further.
The object of the present invention is to provide a material sheet that can be easily produced and having at least similar properties, in particular similar antiballistic properties, as the material known from EP 1627719 A1 or other commercially available materials based on unidirectional PE fibers.
This object is achieved according to the invention by providing a material sheet comprising a woven fabric of polymeric tapes, wherein the width of a tape varies less than 2% on average in the longitudinal direction of the tape. The tapes of the invention having such low variance of width in the longitudinal direction are preferably made by a process which comprises forming a polymeric powder bed, compression-moulding the polymeric powder bed at a temperature below the melting point of the polymeric powder, and preferably drawing the compression-moulded polymer, and wherein the powder bed is moulded by compression together with at least one compressible bordering means positioned onto the powder bed.
In one embodiment of the present invention, there is provided a process for the preparation of a material sheet comprising:
(a) providing a plurality of drawn polymer tapes, preferably unidirectional tapes, having a width that varies less than 2% on average in the longitudinal direction of the tape;
(b) weaving said plurality of drawn polymer tapes to form a woven fabric;
(c) compressing the thus formed woven fabric at least over an area adjacent to the longitudinal edges of the woven fabric to consolidate the area.
In another embodiment, the process is characterised in that, prior to step (c) the unidirectional tapes of the drawn polymer in the warp and weft direction of the woven fabric are at least partly adhered to each other, at least over an area adjacent to the longitudinal edges of the woven fabric, an example of which is depicted in the figures. In still another preferred process according to the invention, adhering the unidirectional tapes is performed by fusion bonding, and even more preferably by ultrasonic welding.
The invention moreover relates to the use of the tapes of the invention in woven material sheets and also in a weaving process for manufacturing material sheets.